The present invention relates to a process for the production of high purity phosphoric acid from crude phosphoric acid solutions such as those produced in a wet process phosphoric acid industry.
If the acid is to be used to produce super phosphoric acid and further to be used for making liquid fertilizers such as 10-34- or 11-37-0, etc. it is essential that the magnesium content of the acid be low. This is because a high MgO content in the super phosphoric acid will cause a gel to form during the preparation of the mixed fertilizer and it will maintain its storage ability. Further, these impurities cause equipment clogging when it is attempted to disperse them in the normal course of their use.
The phosphoric acid produced by conventional leaching operations contains solids and is normally brown or black in color due to the presence of organics or green if the organic colorants are in some way eliminated from the acid such as by prior calcination of the ores. When it is attempted to concentrate the acid from its normal production level of 30 percent P.sub.2 O.sub.5 to 40 or 54 percent P.sub.2 O.sub.5, a considerable amount of calcium sulfate, and other solids are generated in the form of sludge. To make some use of the sludge, the contained aluminum and iron phosphates may be fed to solid fertilizer plants making diammonium phosphate and triple super phosphate. The quantity of this sludge, however, may be in excess of market demand for those products. This can, therefore, place limits on a wet process plant from both a capacity and balance of products standpoint.
Even though the sludges are removed, wet process phosphoric acid still contains considerable impurities which scale process equipment and continue to precipitate from the acid during storage and shipment. In addition, a considerable amount of the fluoride which was present in the rock appears in the acid. If the acid is to be used for the production of animal feeds, the fluoride must, in some way, be removed and this represents a costly additional operation, along with a fluoride pollution problem.
U.S. Pat. No. 3,367,749, entitled "Purifying Phosphoric Acid Using an Amine Extractant", E. L. Koerner and E. Saunders issued on Feb. 6, 1968 discloses a process whereby aqueous impure wet process phosphoric acid is contacted with an amine-solvent system to extract the phosphoric acid into a tertiary amine-solvent system leaving the objectionable metallic impurities behind in the raffinate thereafter the phosphoric acid contained in the amine-solvent system is removed from this phase by methods which include back extracting with water, a suitable aqueous acidic solution, or a suitable basic solution. This process also provides for regeneration of the extractant phase for recycle in their process. However, the aqueous acidic solutions disclosed in the patented process contemplate the use of strong mineral acids which react with the amine portion of the amine-solvent system and form a salt thereof. To regenerate the amine from the salt thereof a stronger base than the amine is required, i.e., ammonia or stronger. The strong base is reacted with the amine salt resulting from the back extraction, thereby producing the free amine and the mineral acid salt. The mineral acid and the strong base are both consumed rendering additional high costs to the process which do not favor process economics.
The art has been seeking a fully cyclic process wherein raw materials are not consumed.